Tracing GC in rust

[elliottslaughter]

Write a tracing garbage collector in rust. Requires various modifications to LLVM and rustc.

LLVM currently has no understanding of registers as GC roots, and needs to augmented with a "gcregroot" intrinsic to declare register roots.

To implement the GC in rust, we need, among other things, for upcalls to be written in rust (#2861).

[lkuper]

Should tracing GC appear on the language development roadmap? (Or is the roadmap only for more user-facing features?)

[graydon]

Quite possibly, in the memory-model section. It's not guaranteed to be user-invisible, and will certainly change performance in a few ways (I hope somewhat drastically).

[graydon]

took this over for 0.7. there's a conservative gc on my gc branch. trying to land presently.

[lkuper]

/me cheers @graydon on

[Aatch]

@graydon you going to be able to land the GC for 0.7?

[graydon]

Unlikely, unless major magic occurs

[graydon]

Moving to 0.8. I was intending it but it isn't going to land. Sorry.

[metajack]

triage bump. i hope this lands in 0.8!

[brson]

Not 0.8. Nominating.

[catamorphism]

accepted feature-complete

[pnkfelix]

cc me

[pnkfelix]

Accepted P-low.

[thestinger]

Closing in favour of #11778, because the two previous false starts here are a distraction from a new implementation.